1. Field of the Invention
The method and apparatus of the present invention generally relates to a method for reading optically recorded data and, more particularly, to a method for compensation for the skew between the scan line of the sensor used to read the recorded data and the recording path line of the data.
2. Description of the Prior Art
Digital information can be recorded using available technology to either burn or not burn small holes in a reflective surface. The information, so written, can be retrieved by detecting the presence or absence of these small holes as the reflective surface is scanned by a reading element. Current technology makes it possible to achieve very high information densities in the order of 200 million dots per square inch. When information is stored on disks which are read when the disks are rotating, the information is usually recorded in circular tracks one dot wide so that at any moment only a single dot is being scanned when the information is being read back from the disk. On optical disks, these circular tracks are selected and sectors are read using techniques that are similar to those used for reading magnetic disks. The high rotational speed of the optical disk combined with the high density of the information in a track produces high data transfer rates when the information is read.
If the same general optical recording technology is applied to tape or data cards, such as credit cards having a strip of optically recorded data, it would be advantageous to be able to scan across the optical media as is done with magnetic tape devices in data processing systems. One such data card having an optical media is described in U.S. Pat. No. 4,360,728, which is incorporated herein by reference.
FIGS. 1 and 2 illustrate such a data card in which card 10 is to have a size common to most credit cards. The width dimension of such a card is approximately 54 mm and the length dimension is approximately 85 mm. These dimensions are not critical, but preferred because such a size easily fits into a wallet and has historically been adopted as a convenient size for automatic teller machines and the like. The card's base 11 is a dielectric, usually a plastic material such as polyvinyl chloride, polycarbonate or similar material. The surface finish of the base should have low specular reflectivity, preferably less than 10%. Base 11 has a shallow groove which carries strip 12. This strip 12 is about 16 millimeters wide and extends the length of the card. The strip is relatively thin, approximately 100-500 microns, although this is not critical. The strip may be applied to the card by any convenient method which achieves flatness. The strip is adhered to the card with an adhesive and covered by a transparent laminating sheet 13 which serves to keep strip 12 flat, as well as protecting the strip from dust and scratches. Sheet 13 is a thin, transparent plastic sheet laminating material or a coating, such as a transparent laquer.
The opposite side of base 11 may have user identification indicia embossed or printed on the surface of the card. Other indicia such as card expiration date, card number and the like may be optionally provided.
The high resolution optical recording material which forms strip 12 may be any of the reflective recording materials which have been developed for use as direct read-after-write (DRAW) optical disks, so long as the materials can be formed on this substrate. An advantage of reflective materials over transmissive materials is that the read/write equipment is all on one side of the card and automatic focus is easier.
With reference to FIG. 3, a magnified view of laser writing on the optical recording strip 12 may be seen. The dashed line 14 corresponds to the dashed line 14 in FIG. 1. The oblong pits 15 are aligned in a path (track) and are generally circular or oval in shape with the axis of the oval parallel to the lengthwise dimension of the strip. A second group of pits 16 is shown aligned in a second track. The pits 16 have similar dimensions to the pits 15. The spacing between tracks is not critical, except that the optics of the readback system should be able to easily distinguish between tracks.
Presently, in optical disk technology, tracks which are separated by only a few microns may be resolved. The spacing and patterns of the pits along each track is selected for easy decoding. For example, oval pits of the type shown can be clustered and spaced in accordance with self-clocking bar codes. If variations in the dimensions of a pit are required, such dimensions can be achieved by clustering pits. The pits illustrated in FIG. 3 have a size of approximately 5 microns by 10 microns and are spaced apart within the track and between tracks.
One difficulty with scanning across the media in tape or data cards is the problem of skew between the track of the recorded data and the reading scan line. The very high packing densities achievable with optical technology make this problem acute. For example, if 5 by 10 micron pits were used to record data across a 1/2 inch tape, the maximum skew allowable between the path used when the data was written and the path of the scan used to read the data is less than one-twentieth (1/20) of a degree. This is illustrated in FIG. 4 which shows the skew angle between the path used to record the data and the path used to scan the data when it is being read. This problem becomes more acute for data cards because the desire to have a low cost reader limits the ability to correct for skew by positioning the card or read mechanism to align the recorded data with the reader.
Therefore, what is needed is a low cost method to correct for skew when reading optically recorded data.